Conceptual groundwater flow model of the Mekelle Paleozoic–Mesozoic sedimentary outlier and surroundings (northern Ethiopia) using environmental isotopes and dissolved ions

Girmay, Ermias; Tenalem Ayenew Tegaye,; Sayfu Kabada,; Alene, Mulugeta; Wohnlich, Stefan; Wisotzky, Frank

Conceptual groundwater flow model of the Mekelle Paleozoic–Mesozoic sedimentary outlier and surroundings (northern Ethiopia) using environmental isotopes and dissolved ions | Modèle conceptuel hydrogéologique d’écoulements des formations sédimentaires Paléozoique–Mésozoique de Mekelle et environs (nord de l’Éthiopie) par l’utilisation des isotopes environnementaux et ions dissous Modelo conceptual de flujo de agua subterránea en el afloramiento sedimentario del Paleozoico–Mesozoico de Mekelle y alrededores (norte de Etiopía) usando isótopos ambientales e iones disueltos 利用环境同位素和溶解离子建立(埃塞俄比亚北部)Mekelle古生代–中生代沉积外露层及周边地区地下水流概念模型 Modelo concetual de fluxo de água subterrânea do afloramento sedimentar Paleo–Mesozóico de Mekelle e imediações (norte da Etiópia) através do uso de isótopos ambientais e de iões dissolvidos

2015

A wide range of lithologic units and tectonic disturbances by cross-cutting faults and folds has resulted in the quite complex hydrogeological setting of the sedimentary outlier and its surroundings at Mekelle, northern Ethiopia. The environmental isotopes of oxygen and hydrogen and patterns of dissolved ion concentrations in the groundwater, coupled with understanding of the three-dimensional geological framework, are used to conceptualize the groundwater flow model and recharge–discharge mechanisms in the area. In agreement with the piezometric-surface map, recharge areas are determined to be the highlands (northwest, north, east and south of the study area), characterized by relatively more depleted isotopic compositions, higher d-excess, and lower concentrations of dissolved ions in the groundwater samples; the narrow major river valleys of Giba, Illala, Chelekot and Faucea Mariam are discharge areas. The groundwater divide between the Tekeze and the Denakil basins coincides with the surface-water divide line of these two basins. In most cases, groundwater feeds the semi-perennial streams and rivers in the area. However, isotopic signatures in some wells indicate that there are localities where river flow and seepage from micro-dams locally feed the adjacent aquifers. The lithostratigraphic, geomorphologic, isotopic and hydrochemical settings observed in this study indicate that three groundwater flow systems (shallow/local, intermediate and deep/semi-regional) can exist here. Tritium data indicate that the groundwater in the study area has generally short residence time and is dependent on modern precipitation.

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